Modifying existing messages is a pain, certainly if this has to be done in an automated fashion. The problems are especially had when multiparts have to be created or removed. The rebuild() method tries to simplify this task and add some standard features.

Reconstruct an existing message into something new. Returned is a new message when there were modifications made, undef if the message has no body left, or the original message when no modifications had to be made.

Examples of use: you have a message which only contains html, and you want to translate it into a multipart which contains the original html and the textual translation of it. Or, you have a message with parts flagged to be deleted, and you want those changes be incorparted in the memory structure. Another possibility: clear all the resent groups (see Mail::Message::Head::ResentGroup) from the header, before it is written to file.

Reconstructing is a hazardous task, where multi level multiparts and nested messages come into play. The rebuild method tries to simplify handing these messages for you.

The standard set of rules, which is the default for the rules option, is a moderest setting. In stead of copying that list into a full set of rules of your own, you can also specify only some additional rules which will be prependend to the default rule set.

The order of the rules is respected, which means that you do not always need to rewrite the whole rule is (see rule option). For instance, the extra rule of removeDeletedParts returns an undef, which means that it cancels the effect of the default rule replaceDeletedParts.

The message-id is an unique identification of the message: no two messages with different content shall exist anywhere. However in practice, when a message is changed during transmission, the id is often incorrectly not changed. This may lead to complications in application which see both messages with the same id.

The ARRAY is a list of rules, which each describe an action which will be called on each part which is found in the message. Most rules probably won't match, but some will bring changes to the content. Rules can be specified as method name, or as code reference. See the "DETAILS" chapter in this manual page, and recursiveRebuildPart().

By default, only the relatively safe transformations are performed: replaceDeletedParts, descendMultiparts, descendNested, flattenMultiparts, flattenEmptyMultiparts. In the future, more safe transformations may be added to this list.

Rules are method names which can be called on messages and message parts objects. The ARRAY can also list code references which can be called. In any case, each rule will be called the same way:

$code->(MESSAGE, PART)

The return can be undef or any complex construct based on a Mail::Message::Part or coerceable into such a part. For each part, all rules are called in sequence. When a rule returns a changed object, the rules will start all over again, however undef will immediately stop it.

Modifying an existing message is a complicated job. Not only do you need to know what you are willing to change, but you have to take care about multiparts (possibly nested in multiple levels), rfc822 encapsulated messages, header field consistency, and so on. The rebuild() method let you focus on the task, and takes care of the rest.

The rebuild() method uses rules to transform the one message into an other. If one or more of the rules apply, a new message will be returned. A simple numeric comparison tells whether the message has changed. For example

print "No change"
if $message == $message->rebuild;

Transformation is made with a set of rules. Each rule performs only a small step, which makes is easily configurable. The rules are ordered, and when one makes a change to the result, the result will be passed to all the rules again until no rule makes a change on the part anymore. A rule may also return undef in which case the part will be removed from the (resulting) message.

This sections describes the general configuration rules: all quite straight forward transformations on the message structure. The rules marked with (*) are used by default.

descendMultiparts (*)

Apply the rules to the parts of (possibly nested) multiparts, not only to the top-level message.

descendNested (*)

Apply the rules to the message/rfc822 encapsulated message as well.

flattenEmptyMultiparts (*)

Multipart messages which do not have any parts left are replaced by a single part which contains the preamble, epilogue and a brief explanation.

flattenMultiparts (*)

When a multipart contains only one part, that part will take the place of the multipart: the removal of a level of nesting. This way, the preamble and epilogue of the multipart (which do not have a meaning, officially) are lost.

flattenNesting

Remove the message/rfc822 encapsulation. Only the content related lines of the encapsulated body are preserved one level higher. Other information will be lost, which is often not too bad.

removeDeletedParts

All parts which are flagged for deletion are removed from the message without leaving a trace. If a nested message is encountered which has its encapsulated content flagged for deletion, it will be removed as a whole.

removeEmptyMultiparts

Multipart messages which do not have any parts left are removed. The information in preamble and epiloge is lost.

removeEmptyBodies

Simple message bodies which do not contain any lines of content are removed. This will loose the information which is stored in the headers of these bodies.

replaceDeletedParts (*)

All parts of the message which are flagged for deletion are replace by a message which says that the part is deleted.

This section describes the rules which try to be smart with the content. Please contribute with ideas and implementations.

removeHtmlAlternativeToText

When a multipart alternative is encountered, which contains both a plain text and an html part, then the html part is flagged for deletion. Especially useful in combination with the removeDeletedParts and flattenMultiparts rules.

textAlternativeForHtml

Any text/html part which is not accompanied by an alternative plain text part will have one added. You must have a working Mail::Message::Convert::HtmlFormatText, which means that HTML::TreeBuilder and HTML::FormatText must be installed on your system.

If you have designed your own rule, please consider contributing this to Mail::Box; it may be useful for other people as well.

Each rule is called

my $new = $code->($message, $part, %options)

where the %options are defined by the rebuild() method internals. At least the rules option is passed, which is a full expansion of all the rules which will be applied.

Your subroutine shall return $part if no changes are needed, undef if the part should be removed, and any newly constructed Mail::Message::Part when a change is required. It is easiest to start looking at the source code of this package, and copy from a comparible routine.